The Cooke CVE 301 is a low cost ($12K) one button pump down vacuum evaporator. Although it can pump down from a standby state with the push of a button, you will be operating the system in manual mode. This means you must have an understanding of how the system works.

The basic concept of a diffusion pump is to remove the atmosphere that diffuses into jets of boiling hot oil directed away from the chamber. The momentum transfer from the diffusion pump oil molecules to the atmospheric particles carries the undesireable atmosphere away so it cannot contaminate the film to be deposited. The pumping mechanism is not unlike the cause of the wind that one feels at the bottom of a waterfall on a calm day.

Complications arise from the fact that no diffusion pump oil known is capable of pumping from atmospheric pressure to more than eight orders of magnitude below. The problem is oxidation of the pump oil. It proceeds at an unacceptable rate at pressures above about 10-4 atmospheres. Therefore a mechanical pump must be employed to keep the pressure at either end of the diffusion pump below 100 microns when the diffusion pump oil is hot. Your responsibility will be to make sure the valves are sequenced properly to keep the oil from "cracking" (a term for rapid oxidation of pump oil).

The special property that distinguishes diffusion pump oil from other oils is that it can be recycled within the diffusion pump. See figure 1 in the paper version. Water cooled side walls of the pump body condense virtually all the diffusion pump oil. The oil is then re-boiled (high vapor pressure) at the base of the diffusion pump while the mechanical pump removes other particles swept down by the oil jets.

Diffusion pump oil does have a measurable vapor pressure of its own, even at the cooling water temperature. The price goes up as that vapor pressure goes down. To keep diffusion pump oil from diffusing up into the chamber, first a water cooled (chevron) baffle and then a liquid nitrogen cooled "trap" are used to contain the oil. Both are "optically dense" meaning that light or particles with a mean free path greater than the system dimensions must collide with at least one surface. The cool surfaces will condense the majority of diffusion pump oil. The 77 ° Kelvin walls of the liquid nitrogen trap also enhance pumping speed by literally freezing out some of the atmosphere and compacting the rest (PV=nRT), a phenomenon called cryopumping.

Thin film deposition can be done by many methods. You will use the simplest. Once the atmosphere is sufficiently removed for a clean deposition, aluminum will be boiled by an electrically heated filament coating everything within sight of it. Aluminum has the useful property of clinging to (or "wetting") the relatively inexpensive tungsten filaments when it melts rather than falling through.